Having everyone on GMail is not a good thing.
The reality, from what I’ve gleaned, is that the instance you’re on matters (for many), but you can’t come to appreciate that and what you want in an instance without first being on some instance and active on Mastodon/fediverse.
So I think the solution is that there is essentially a new-to-mastodon instance or domain (of multiple servers that all work the same). It’s for people feeling out mastodon and exploring. And there can even be a time limit for accounts there to prevent accounts sitting around forever.
That way there’d be one domain to point everyone to but the uniqueness of instances and the diversity of the fediverse can be maintained.
Main thing that needs to be cleaned up though is moving instance. It’s not hard now, but this idea would be more attractive if it was one-button-easy.
More generally, the portability of accounts across the fediverse is something I really think should be more of a thing.
My advice has been to just pick an instance and try it out, and then migrate to a different one after a month if they see one that looks like a better fit.
This is the wrong question to ask. Last I used Twitter it bombarded me with unsolicited, unrelated content to an inescapable degree. On Mastodon, which so enthusiastically compensates for Twitter’s aggression that no new content is surfaced without an explicit request or first-degree vouch, I take a peek at instances' local timelines almost daily.
Beyond the timeline, consider the behavior of hashtags. On a single-user instance hashtags are useless for discovery.